We have been studying the induction of histocompatibility-gene mutations in the spermatogonia of BALB/c mice, treated with X-rays. The treated males are mated with C57BL/6 females, and their F1 hybrid progeny are screened for a change in histocompatibility phenotype. The results have indicated the system to be m 60 times more resistant than predicted by the results of the 7-locus mutation-rate test. We are now studying the recovery of mutants when the treated males are mated with BALB/c females. We next plan to study the recovery of histocompatibility mutants following irradiation of BALB/c sperm, and for comparison, after treatment with EMS. The mutants recovered in these experiments are being used to establish congenic or coisogenic mutant lines. BIBLIOGRAPHIC REFERENCES: Melvold, R. W., and H. I. Kohn (1975) Histocompatibility gene mutation rates: H-2 and non-H-2. Mutation Res. 27:415-418. Kohn, H.I. and R. W. Melvold (1976) Divergent X-ray-induced mutation rates in the mouse for the "H" and "7-locus" groups of loci. Nature 259: 209-210.